U.S. patent application Ser. No. 386,381, filed July 28, 1989, Mosehauer et al, shows an electrophotographic apparatus in which an electrostatic image is toned on the same area that already contains a loose toner image of a different color to create a multicolor image. The multicolor image is then transferred to a receiving sheet at a single transfer station. A large number of other references show this basic process; see, for example, U.S. Pat. Nos. 4,308,821; 4,629,669; 4,599,285 and 4,731,634.
This process eliminates registration problems at the transfer station common to structures in which separate toner images are created consecutively on an imaging member and then transferred to a recieving sheet that is brought into transfer relation with those images repetitively. However, in this process, registration between separate electrostatic image forming mechanisms must be maintained. The structure shown in the Mosehauer et al application requires both in-track and cross-track registration between LED printheads facing an imaging web supported by backing rollers.
The imaging web has the advantage of facilitating receiving sheet handling. As shown in that application, the web is trained around several small rollers which cause a change in direction of the web that facilitates separation of the receiving sheet. "Single-pass duplexing" is facilitated by the web.
However, all webs have tracking problems. Maintenance of cross-track and in-track registration between toner images is a challenge in designing and manufacturing color apparatus having a web imaging member. In-track registration is controlled in the Mosehauer et al structure by a sprocket and perforation system which automatically adjusts for variations in the speed and manufacture of the web. Reasonably good cross-track registration is obtained since the web has a tendency to follow itself over short distances reasonably well. However, for highest quality work even complex web tracking mechanisms are unable to prevent some noticeable cross-track misregistration. This becomes more critical as materials and exposure systems are improved and provide higher resolution.
U.S. Pat. Nos. 4,378,154; 4,624,549; 4,497,570 and 3,820,985 show an electrophotographic imaging apparatus in which a photoconductive web is trained around a large roller or drum and at least one or more small rollers. A small roller has the advantage shown, for example, in U.S. Pat. No. 4,378,154, of assisting in separation of a receiving sheet. The large roller or drum helps maintain good separation between image forming stations and the electrophotographic member.
FIG. 4 of U.S. Pat. No. 3,820,985 shows such a drum-web structure in a color imaging device. Registration problems are not addressed or corrected in this structure.